1. Field of Invention
This invention relates to rejuvenating permselective ion-exchange membranes employed as selective barriers between the anolyte and catholyte of brine electrolysis cells.
2. Definitions Used Herein
"Carbon oxide" is used herein to mean carbon dioxide, or carbonic acid, or a carbonate or bicarbonate of an alkali metal or an alkaline earth metal (including magnesium), or a combination of any of these.
"Cathodic protection voltage" is defined herein to mean a cell voltage drop, as measured between the anode to the cathode of a cell, which is just large enough to cause reduction of water to hydrogen and hydroxyl ions at the cathode. Such a cell voltage is, therefore, capable of providing cathodic protection for the cathodes to prevent them from corroding.
3. Discussion of Prior Art
The electrolysis of chlorides of monovalent cations (including lithium, sodium, potassium, rubidium, cesium, thallium and tetra methyl ammonium) with cation selective membranes is well known for the production of chlorine and the hydroxides of such cations, particularly with respect to the conversion of sodium chloride to chlorine and caustic. Representative of such permselective cation exchange membranes are the perfluorosulfonic acid membranes made and sold by the E. I. duPont de Nemours & Co., Inc., under the tradename, Nafion, and the perfluorocarboxylic acid membranes of the Asahi Glass Industry Co., Ltd. of Tokyo, Japan. See U.S. Pat. No. 4,065,366 to Oda et al for a description of the latter carboxylic acid type membranes.
In the process of electrolyzing sodium chloride into chlorine and caustic wherein such membranes are used, the membrane divides the cell into anode and cathode compartments. Brine is fed to the anode compartment and water is fed to the cathode compartment. A voltage impressed across the cell electrodes causes the migration of sodium ions through the membrane into the cathode compartment where they combine with hydroxide ions (created by the splitting of water at the cathode) to form an aqueous sodium hydroxide solution (caustic). Hydrogen gas is formed at the cathode and chlorine gas at the anode unless a depolarized cathode is used. (When a depolarized cathode is used, H.sub.2 gas is not generated.) The caustic, hydrogen and chlorine may subsequently be converted to other products such as sodium hypochlorite or hydrochloric acid.
It is known that over a long period (&gt;100 days) of use of such membrane-type cells, there occurs an undesirable increase in the cell voltage and electrical energy consumed per unit (e.g. ton) of product made. The prior art in general has attributed this undesirable increase to the fouling of the membrane by hardness and other multivalent cation impurities contained in the brine feed..sup.1 The calcium cation in particular has been singled out as the most damaging impurity. FNT .sup.1 See U.S. Pat. No. 3,793,163 to R. S. Dotson (1974); The Asahi Chemical Membrane Chlor-Alkali Process, page 5 of a paper presented by Maorni Seko of Asahi Chemical Industry Co., Ltd., of Tokyo, Japan, at The Chlorine Institute, Inc., 20th Chlorine Managers Seminar, at New Orleans, Louisiana on Feb. 3, 1977; Effect of Brine Purity on Chlor-Alkali Membrane Cell Performance, a paper originally presented by Charles J. Molnar of E. I. duPont de Nemours & Co., Inc., and Martin M. Dorio of Diamond Shamrock Corporation at The Electrochemical Society Fall Meeting held October, 1977, at Atlanta, Georgia; The Commercial Use of Membrane Cells in Chlorine/Caustic Plants, pages 6-9 of a paper presented by Dale R. Pulver of Diamond Shamrock Corporation at The Chlorine Institute's 21st Plant Manager's Seminar, at Houston, Texas, on Feb. 15, 1978; Nafion.RTM. Membranes Structured for High Efficiency Chlor-Alkali Cells, a paper presented by Charles J. Hora of Diamond Shamrock Corporation and Daniel E. Maloney of E. I. duPont de Nemours & Co., Inc., at The Electrochemical Society Fall Meeting, October, 1977, Atlanta, Georgia; U.S. Pat. No. 4,115,218 to Michael Krumpeit (1978); U.S. Pat. No. 4,073,706 to Zoltan Nagy (1978); U.S. Pat. No. 3,988,223 to S. T. Hirozawa (1976); U.S. Pat. No. 4,204,921 to W. E. Britton et al (1980); U.S. Pat. No. 4,202,743 to Oda et al (1980); and U.S. Pat. No. 4,108,742 to Seko et al (1978).
To prolong the useful life of these membranes many techniques have been developed to reduce the amount of contaminants in the brine which foul the membrane. Many of the references cited above give methods for further reducing the multivalent cation impurities contained in the cell's feed brine. A very recent technique discovered for reducing membrane fouling centers around using brine which contains very little carbonate anions or carbon dioxide (carbon oxides) in the feed brine. This technique is disclosed in a recently filed patent application entitled, "Membrane Cell Brine Feed", having Ser. No. 248,670 and a filing date of Mar. 30, 1981, and having as inventors Bobby Ray Ezzell and Harry Stevens Burney, Jr. The latter named inventor is a co-inventor of the instant invention, and this prior patent application is incorporated by reference herein as is set forth at length for purposes of prior art teachings and for the new techniques taught therein pertaining to obtaining improved brine for electrolysis in an electrolytic cell which employs a permselective membrane disposed between the anode and cathode.
To further prolong the life of these permselective membranes, several techniques for regenerating them in place have been developed. For example, U.S. Pat. No. 4,115,218, by Michael Krumpelt (issued Sept. 19, 1978) teaches that such membranes can be rejuvenated by merely reducing or interrupting the cell current or voltage alone or in combination with a concomitant flushing of the catholyte portion of the cell. This process is limited to the instance where the brine fed to the cell during its normal operation contains a calcium content which is less "than is ordinarily used".
Another example of membrane regeneration is found in U.S. Pat. No. 3,988,223, by Stanley T. Hirozawa (issued Oct. 26, 1977). This patent teaches unplugging the membrane by a process which comprises maximizing the brine head, adding a chelate or chelate forming agent to the anolyte, shunting the electrical current to the cell, flushing the cell, and removing the shunt.
A third example of membrane regenerating is found in U.S. Pat. No. 4,040,919, by Jeffrey D. Eng (issued Aug. 9, 1977). This patent teaches these membranes can be regenerated by increasing the acidity of the anolyte, diluting the electrolyte located immediately adjacent to the anolyte and separated from the anolyte by a membrane, reducing the current density, and maintaining such conditions during electrolysis for a period sufficiently long to rejuvenate the membrane. Note, usually the electrolyte referred to in this patent can be the catholyte, but it does not have to be. It can be an electrolyte located between two spaced membranes which are both located between an anode and a cathode.
These membrane regenerating techniques are an improvement over the alternative of replacing the membranes, but only marginally so in many instances. Generally these techniques produce only a short term improvement, particularly short term improvements insofar as are concerned the cell voltage and cell energy requirement (unit of energy used to make a unit of cell product).
It is not certain why these membrane regenerating techniques usually produce only short term improvements, but it seems in accordance with the discovery of the present invention that these techniques can readily remove some salts from the membrane, but can remove substantial amounts of impregnated calcium carbonate only at the expense of doing considerable damage to the membrane. It would be advantageous to overcome these deficiencies, and the method of the present invention at least partially does.